In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (having a thickness of approximately several nm to several hundred nm) formed over a substrate having an insulating surface is attracting attention. Thin film transistors are widely applied to electronic devices such as an IC or an electronic optical device. In particular, the development of a thin film transistor as a switching element in an image display device is advanced rapidly.
Conventionally, in an active matrix liquid crystal display device driven by TFTs, a large number of scanning lines and data lines are provided vertically and horizontally over a substrate and a large number of TFTs are provided in accordance with the intersections of these wirings. In each TFT, a gate wiring is electrically connected to the scanning line, a source electrode is electrically connected to the data line, and a drain electrode is electrically connected to a pixel electrode.
In a transmissive liquid crystal display device, ITO which has both light-transmitting properties and electric conductivity is commonly used for the pixel electrode. This pixel electrode and metal wirings such as a data line and a scanning line are insulated by a layer made of an insulating material. The pixel electrode and the metal wirings are in contact with each other through a contact hole formed at a particular location in the insulating film.
As the area of a display screen becomes larger, the delay of a signal due to the resistance of wirings becomes a more notable problem. Therefore, it is necessary to drastically change the shape of a wiring and an electrode or to use a low-resistant material such as aluminum for the wiring and the electrode.
When aluminum used as the material of the wiring and the electrode contacts ITO used as the material of the pixel electrode, a reaction called electric erosion occurs at the junction interface. Further, when aluminum contacts ITO, the surface of aluminum is oxidized and becomes electrically non-conductive.
Consequently, in order to prevent the electric erosion when the wiring, the electrode, and the like formed with such two incompatible films are connected, a technique has been suggested in which a metal film having a high melting point (such as a titanium film), a metal compound film having a high melting point (such as a titanium nitride film), or the like is provided between ITO and an aluminum wiring (or electrode) to avoid the electric erosion with ITO.
The present applicant describes in Patent Documents 1 to 3 that a drain of a thin film transistor and ITO serving as a pixel electrode are connected by sandwiching therebetween a multilayer film including a titanium film, an aluminum film, and a titanium film.
Further, the present applicants describe in Patent Document 4 that a drain of a thin film transistor and ITO serving as a pixel electrode are connected by sandwiching therebetween a multilayer film including a titanium film and an aluminum film. Moreover, the present applicants describe in Patent Document 5 that a drain of a thin film transistor and ITO serving as a pixel electrode are connected by sandwiching therebetween a multilayer film including a titanium nitride film and an aluminum film.
The present applicant also describes in Patent Document 6 that a gate electrode of a thin film transistor is formed with two layers having different widths so as to form a GOLD structure.
[Patent Document 1]
                Japanese Published Patent Application Laid-Open No.: H9-45927[Patent Document 2]        Japanese Published Patent Application Laid-Open No.: H10-32202[Patent Document 3]        Japanese Published Patent Application Laid-Open No.: H6-232129[Patent Document 4]        Japanese Published Patent Application Laid-Open No.: 2004-6974[Patent Document 5]        Japanese Published Patent Application Laid-Open No.: H8-330600[Patent Document 6]        Japanese Published Patent Application Laid-Open No.: 2001-281704        